In the sport of offroading almost inevitably the
conversation turns to improving performance. When offroad an
important keyword is traction. Better traction generally boils down
to four components, your tires. Quality tires designed for offroad
make a world of difference in your ability to find traction and have fun.
But if a tire is not turning due to a lack of power being sent to that
tire, the best tires in the world won't help you. That's where the
right differential makes a difference. This article will
attempt to explain what role your differential plays in traction and the
different types of differentials available on the market and how they will
affect your vehicle's traction and handling characteristics.

The
Differential

The differential in a vehicle is located in what is
sometimes called the "pumpkin", or that center section of the front or
rear axle that intersects with the drive shaft. Within that center
part of the axle is contained the differential.

In a vehicle, the differential usually consisting of a set of gears,
that allows each of the driving wheels to rotate. The gears convert
the rotating motion of the driveshaft or drive train and split power to
each of the driving axle shafts of that axle. In 4 wheel drive
vehicles there are two differentials, one in the rear axle and one in the
front axle.

The
differential has three jobs. It directs engine power to the wheels.
It acts as the final gear reduction in the vehicle, slowing the rotational
speed of the transmission (and transfer case of 4 wheel drive vehicles)
before it hits the wheels. The differential also transmits the power
to the wheels while allowing them to rotate at different speeds, thus the
term "differential".

The
main purpose of the differential is to allow each half of the axle (each
tire) to spin at different speeds, while supplying an equal amount of
force to each wheel in that axle. The need for the wheels to rotate
at different speeds is especially apparent when turning corners.
When cornering the inner wheel travels a shorter distance than the outer
wheel. With an open differential they both propel the vehicle
forward with equal force, so long as both wheels remain in contact with
the road and have traction. However if one wheel slips, for instance
on ice, more torque is sent to the wheel that spins. If that slipping
wheel completely looses traction, all power will be sent to that wheel and
you have no forward momentum. When offroad, this
is where the common open differential fails to remain effective.
When offroad there are many situations where a wheel will spin free.
In most stock 4x4 vehicles the common Open Differential can be found in
both the front and rear axles. When a wheel in the front AND a wheel
in the back are allowed to spin free due to the Open Differentials, that
4x4 is essentially a 2 wheel drive vehicle. One front wheel, and one
back wheel. This is where other types of differentials will make
drastic improvements to traction.

Types of
Differentials

Differentials can be generally
classified into 4 categories. Open Differentials, Limited Slip
Differentials, Locking Differentials and Spools. Spools are really
just the elimination of the differential, so really, there are three
categories.

Beyond the open differential, the
various types of "non-open" differentials will provide varying degrees of
limiting of the spin or slip of an open differential. What also
varies is the feel of these differentials, which translates into varying
degrees of handling characteristics on road and offroad.

Open / Standard Carrier
Differential

The standard differential, or what is referred to as an open carrier, is
what comes with most OEM vehicles. The open carrier holds the ring
gear in place and within the open carrier is generally a set
of gears called spider gears. These spider gears are responsible for
allowing a vehicle to negotiate a turn and allow the outside wheel
to travel farther
and turn faster than the inside wheel. This type of open design
works great for most of vehicles on the road today. However when a
vehicle with an open differential meets a lack of traction, it directs
power to the wheel with the least amount of resistance. The result is the
wheel on the traction-less surface spins free, while the opposite wheel of
that axle on the better traction surface provides little or no power.

Limited Slip
Differentials, Posi-Traction (Posi, Posis)

Limited Slip and positraction (posi) differentials are
designed to "limit" the tendency of open differential to send power to a
wheel that lacks traction and redirect the power to a degree to the
other wheel of the axle. The Limited Slip and Positraction
differential will send power to both wheels equally when traveling
straight, however when one wheel spins due to a lack of traction, the
differential will automatically provide torque to the other wheel with
traction. Limited Slip and Positraction (posi) differentials limit
the loss of torque to a slipping wheel through various mechanisms such
as clutches, gears cones, and other methods dependant on the unit.
The limited slip and positraction will not provide 100% lock up of the
differential in extreme situations such as when a wheel completely
looses traction. Limited Slip and Positraction (posi)
differentials are recommended for daily driven vehicles and are used in
many applications where traction is sometimes needed as in emergency
vehicles. They are also ideal for front axles of 4x4 vehicles that
are not equipped with front hubs that can be disengaged. The term
"positraction" ("posi" for short) was used by
General Motors years ago for their limited slip differential and has
been used to refer to limited slips since.

CLUTCH-TYPE LIMITED SLIP
GEAR-DRIVEN LIMITED SLIP

Lockers,
Locking Differentials

A locking differential or "Locker"
uses a mechanism that allows left and right wheels to "lock" relative to
each other and turn at the same speed regardless of which axle has
traction and regardless of how little traction a slipping wheel has.
In this state, the axle acts more as a "Spool". This means
traction can be sent to a wheel that may be planted firmly on the ground
while the other wheel of the axle is completely off the ground. In
this situation an open differential will spin the free (lifted) wheel
sending absolutely no torque to the wheel in the ground. A limited
slip in this situation will send some torque to the wheel on the ground
but possibly no enough to provide any forward momentum.

Lockers use various mechanisms to
provide lock-up and can be divided into two categories, Automatic
Lockers and On-Command, or selectable Lockers.

Automatic Lockers:

Automatic locking differentials are
designed to lock both wheels of an axle automatically when torque is
applied so that both wheels are providing power. When torque is not
being applied such as when the clutch is press down, the differential is
allowed to unlock, permitting a variance in wheel speed while negotiating
turns. Automatic lockers tend to create odd handling characteristics
on the street as they lock and unlock and take some getting used to.

On-command
lockers are the best of both worlds providing the benefits of a locking
differential and an open differential. An on-command locker uses a switch activated electric motor or
vacuum diaphragm or a cable / lever to engage the locker. When an
on-command locker is not engaged, it acts like a standard open
differential with none of the quirky handling characteristics of an
automatic locker. When the on-command locker is engaged, the
differential locks the axle shafts together where it is now more like a
spool with no differential of speed between the wheels of that axle.
Some OEM on-command locker designs are available on the market including
1998 and newer Toyota Tacoma and Land Cruiser and the Jeep TJ and JK
Rubicons.

Spools are actually the lack of a differential. Spools are a 100%
lock-up between both wheels of an axle all the time. Spools are
generally used for racing and serious offroad use where little or no
street driving is seen by the vehicle and a stronger, lighter rear end is
needed.